The invention relates to circuits employing mechanical resonators in voltage controlled oscillators. Mechanical resonators are often used because they are inherently stable and can be manufactured to relatively precise frequencies. Since such resonators are inherently stable, they are difficult to frequency modulate, particularly if a wide range of modulation is desired. In many cases it is desired to cause an oscillator to track a remotely produced signal. This is commonly done by a voltage control circuit which shifts the oscillator frequency to produce the desired tracking.
U.S. Pat. No. 3,973,221 shows a voltage controlled crystal oscillator of the kind used in the chrominance control in a color television system. The oscillator includes a .+-.45 degree voltage controlled phase modulator. With a balanced or reference input the phase shift is zero degrees and the crystal oscillator operates at its natural series resonance. As the control voltage varies the phase shift over the .+-.45 degree range, the crystal will oscillate at that frequency which causes it to introduce a phase shift of the opposite sign. Thus by means of a phase detector such an oscillator can be caused to operate in phase with the color television color burst.
The wide range of operation, .+-.45 degrees, means that the crystal operates over a substantial portion of its series resonant mode. Typically the phase shift versus frequency is greater on one side of resonance than it is for the other side of resonance. Accordingly, the control sensitivity in terms of frequency shift as a function of voltage is non-linear. While this characteristic is not a problem in many applications, it can have serious consequences in certain applications. For example, in a phase locked loop (PLL) the loop response to transients is a strong function of effective loop gain, which in turn is a function of oscillator voltage control sensitivity. In applications where transient response is important, a non-linear response produces excessive variations in transient response.